Phosphatidylinositol 3-kinase (PI3K), a ubiquitous lipid kinase involved in receptor signal transduction by tyrosine kinase receptors, includes a large and complex family that includes 3 classes with multiple subunits and isoforms (Cantley, Science 296:1655-1657 (2002); Carpenter and Cantley, Curr. Opin. Cell Biol., 8:153-8 (1996)). The class I PI3Ks are composed of a Src homology-2 domain-containing an 85 kDa regulatory subunit (p85) and a 110-kDa catalytic subunit (p110), which catalyze the phosphorylation of phosphoinositol 4-phosphate and phosphoinositol 4,5-phosphate at their D3 position (Cantley, Science 296:1655-1657 (2002); Carpenter and Cantley, Curr. Opin. Cell Biol., 8:153-8 (1996)). The PI3K regulatory subunits include p85α and its truncated splice variants p50α and p55α, as well as p85β and p55γ; the catalytic subunits include p110α, p110β, and p110δ (Cantley, Science 296:1655-1657 (2002)). The regulatory subunits p85α, p50α, and p55α are encoded by the pik3r1 gene; p85α is the most abundantly expressed regulatory isoform of PI3K, and p55α and p50α are 2 additional minor alternative splicing isoforms (Ueki et al., Mol. Cell. Biol. 20:8035-8046 (2000)).
The type I enzymes have been extensively studied and were originally identified in association with tyrosine kinases such as growth factor receptors and products of oncogenes (Khaleghpour et al., Carcinogenesis, 25:241-248 (2004)). Most studies regarding the type I PI3Ks have focused on the a form. In particular, class IA PI3Ks are strongly expressed in colonic epithelial carcinoma cell lines (Shao et al., Cancer Res., 64:229-235 (2004)). The gene coding for p110α (pik3cα) is amplified in ovarian and breast tumors (Campbell et al., Cancer Res., 64:7678-7681 (2004)), implicating pik3cα as a potential oncogene in these cancers. An oncogenic mutated form of p85α has also been described (Jimenez et al., EMBO J., 17:743-753 (1998)), expression of this allele associates with endogenous p110 and increases its activity in a constitutive manner, leading to cell transformation. In addition to the regulation of normal cell processes, the promotion of cell survival by the activation of PI3K occurs by the inhibition of proapoptotic signals and the induction of survival signals, which contribute to the malignant transformation and tumor progression (9). In this regard, there is a growing body of evidence to support the notion that the activation of PI3K/Akt is associated with colorectal carcinoma and can convert differentiated human gastric or colonic carcinoma cells to a less differentiated and more malignant phenotype (Semba et al., Clin. Cancer Res., 8:3824-3831 (2002)). The effects of PI3K on tumor growth and progression are thought to be mediated by Akt, a downstream effector of PI3K (Fresno Vara et al., Cancer Treat Rev., 30:193-204 (2004)). The Akt family defines a family of closely related highly conserved cellular homologs of the viral oncoprotein v-akt (Bellacosa et al., Science., 254:274-277 (1991)). In humans, there are 3 members of the Akt gene family, designated Akt1, Akt2, and Akt3, which are located on different chromosomes. The Akt gene products, cytoplasmic serine/threonine (ser/thr)-specific protein kinases, are major downstream targets of numerous receptor tyrosine kinases signaling via PI3K (Fresno Vara et al., Cancer Treat. Rev., 30:193-204 (2004)). Akt is overexpressed in a number of cancers, including colon, pancreatic, ovarian, and some steroid hormone-insensitive breast cancers (Roy et al., Carcinogenesis, 23:201-205 (2002); Asano et al., Oncogene, 23:8571-8580 (2004)). Moreover, it has been reported that Akt phosphorylation in human colon carcinomas correlates with cell proliferation and apoptosis inhibition, as well as with different clinicopathologic parameters such as invasion grade, vessel infiltration, metastasis to lymph nodes, and tumor stage (Khaleghpour et al., Carcinogenesis, 25:241-248 (2004); Itoh et al,. Cancer, 94:3127-3134 (2002)).
Inhibitors of proteins that are involved in PI3K/Akt signaling have been suggested as potential therapeutic agents. These include inhibitors that target both upstream regulators of PI3K/Akt, such as growth factor receptors, PI3K and Akt inhibitors, and downstream effectors, such as the components of the mTOR pathway. The components of the regulatory system for PI3K/Akt that have proved most amenable to therapeutic intervention are the growth-factor-receptor tyrosine kinases, in particular, the epidermal growth factor receptor (EGFR), its close relative ERBB2, and the fungal metabolite wortmannin, a PI3K inhibitor (Wang et al., Clin Cancer Res. 8:1940-1947 (2002); Hortobagyi, Cancer., 88 (suppl 12):3073-3079 (2000)). Disadvantages of wortmannin include its short half-life, solubility in organic solvents, and toxicity, which limits its use in clinical trials (Gunther et al., Food Chem. Toxicol., 27:173-179 (1998)). An alternative approach to the therapeutic targeting of the PI3K/Akt pathway is to specifically inhibit the expression of important pathway proteins by RNA interference (RNAi). RNAi is an evolutionary conserved mechanism that is operative in insects, nematodes, plants, and mammalian cells (Matzke and Birchler, Nat. Rev. Genet., 6:24-35 (2005)). In this process, sequence-specific posttranscriptional silencing is initiated by the introduction into cells of double-stranded annealed sense and antisense RNAs that are homologous to the sequence of the silenced gene (Matzke and Birchler, Nat. Rev. Genet., 6:24-35 (2005)). Small interfering RNAs (siRNAs) can be targeted to tumors, and several recent studies indicate the potential for application of this technique in the therapy for various cancers (Yin et al., J. Exp. Ther., 3:194-204 (2003); Takeshita et al., Proc. Natl. Acad. Sci., USA, 102:12177-12782 (2005)).